A R.sub.2 T.sub.14 B rare earth permanent magnet represented by a Nd-Fe-B magnet is generally known to have superior magnetic properties to a Sm-Co rare earth permanent magnet. Moreover, consisting of Nd and Fe which are rich natural resources, the former magnet is provided at a lower price than the latter and is being used widely.
On the contrary, however, the R-Fe-B rare earth permanent magnet has a special internal oxidation factor that it includes in its metallic organization of an alloy a R-Fe solid solution which is oxidized extremely easily in the atmosphere. The R-Fe-B rare earth permanent magnet had, therefore, problems that an oxide layer formed at the surface of the magnet by precipitation brought about deterioration and irregularity in the magnetic properties and that being used as such an electric part as a magnetic circuit, the dispersion of the oxide film contaminated the peripheral devices.
To remove the problems, a method has been applied in the prior art for forming an oxidation resistant film such as a plating film or a chemically formed film at the surface of the magnet using a water solution as a plating solution which is disclosed in Japanese patent prepublications Tokkai Sho 60-54406 or Tokkai Sho 60-63903.
The prior method for forming an oxidation resistant film such as a plating film or a chemically formed film described above has, however, a defect that the R-Fe solid solution was rapidly oxidized in the plating process because the method has an outer oxidation factor that large quantity of water or water solution is used as a plating liquid for plating process. As a result, a problem arose that the effect of preparation process which is important in the plating process was lost thereby preventing generation and growth of the plating film at the surface of the magnet bringing about poor adhesion and powder precipitates.
Further, even though the oxidation resistant film such as a plating film or a chemically formed film was provided, oxidation proceeded internally by an oxide layer or absorbed water remaining between the surface of the magnet and the plating film thereby leaving a cause of the poor adhesion such as swell or exfoliation of the film.
Further, in the surface treatment according to the PVD method such as an ion-plating, the film formed was a pastic precipitate lacking fineness.
It has been, therefore, difficult to improve the oxidation resistance by the prior surface treatments.
On the other hand, a method has been known for coating the surface of the rare earth permanent magnet by using an organic electrolyte plating method in which a nonwater organic solvent is used as an electrolyte (a tetrahydrofuran cell etc.). The organic solvent, even if it is a nonwater plating liquid, has, however, a defect which is peculiar to the organic solvent that it deteriorated even its dielectric constant since the organic solvent itself is a polar solvent which easily absorbs water and has small solubility of salts.
Further, to remove the defects described a prior ordinary supporting electrolyte could not cope with the internal oxidation factor which is peculiar to the R.sub.2 T.sub.14 B rare earth permanent magnet material having the R-Fe solid solution extremely easy to be oxidized and thereby obtaining a plating film having no brilliance and poor adhesion.
It is, therefore, a first object of the present invention to provide an organic solvent electrolyte for forming on the surface of a R.sub.2 T.sub.14 B intermetallic compound permanent magnet an oxidation resistant film having an improved brilliance (an appearance) and adhesion by using an organic electrolysis method necessitating no large quantity of water or water solution in a plating process.
Further, it is a second object of the present invention to provide an organic solvent electrolyte for a plating film which uses a supporting electrolyte for wide use which is applicable to various kinds of organic solvents in accordance with the internal oxidation factor which is peculiar to the R.sub.2 T.sub.14 B rare earth permanent magnet.
Further, it is a third object of the present invention to provide an organic solvent electrolyte for a plating film which improves the solubility into the organic solvent and the conductivity of the supporting electrolyte according to the present invention.
Further, it is a fourth object of the present invention to provide an organic solvent electrolyte for a plating film which removes the outer oxidation factor in the plating process.